qmlsc: Only changed files are checked in incremental builds

It makes sense of course. But if there was an issue that qmlsc complained about in one file and fixing it required a change only in some other file, qmlsc isn't run on the first file again.

That causes the user to think it is all fine now, even if the issue wasn't really fixed or there would have been other unrelated issues.

Maybe this could be made to work by making the build to fail if qmlsc finds issues from a file about to be compiled.

Could be basically reproduced by making an error in one file and running build twice, it only complains about it the first time.